Bra Size Calculator

Two tape measurements, one starting point

Every band and cup system starts from the same two numbers: the circumference of your ribcage right under the bust, and the circumference at the fullest point. Wrap a soft tape snugly under the bust, exhale, and read it flat against the skin without squeezing. Then measure around the back and over the fullest part of the bust, keeping the tape level all the way around. Two minutes with a $5 tape gets you a size that fits better than most department store fittings.

Why not skip the tape and trust the size you have worn for years? Because the studies disagree with the labels. A 2008 study in Chiropractic & Osteopathy measured 30 young women and found 80 percent wore an incorrectly sized bra, most of them too small in the cup. A 2013 study of postmenopausal women reported 93 percent incorrectly sized. Sizing that starts from a remembered label inherits every mistake in it.

Treat the calculator's output as the center of a range, not a verdict. Band and cup letters interact, brands run a size apart from each other, and breast shape changes with weight, exercise, pregnancy and age. The tape gives you a place to start trying things on; the mirror finishes the job.

The band: rounding to the nearest even inch

US bands are manufactured in even numbers: 30, 32, 34, 36 and so on. The modern convention rounds the underbust to the nearest inch first, then to the nearest even number, because bands are made in even sizes. An underbust of 31 inches sits exactly between 30 and 32, and the tie rounds up, giving a 32 band. A 31.5 inch underbust also lands on 32, while 33.4 inches rounds to 34. Measuring in centimeters gives the same answer once it is converted and rounded the same way, so 78.7 cm lands on a 32 band too.

You will still meet the older method that adds 4 inches before rounding. It dates from a time when bras were built with non-stretch fabric and needed extra room. Run the same body through both and the gap is striking: an underbust of 29 inches with a 36 inch bust gives a 30 band and a DDD cup under the modern method, but a 34 band and a B cup under the +4 method. Both describe one person. The +4 method survives in older charts and some store fittings, but nearly all current American and British fitting advice uses the round-to-even approach, because modern elastic bands are designed to sit snug and stretch.

A band that fits should fasten on the loosest hook and let no more than two fingers under it. If the tape says 32 but a 32 band rides up, take the sister size in the next section rather than jumping to a different cup letter at the same band.

The cup: one inch per letter

The US cup letter counts inches from band to bust. One inch is an A, two a B, three a C, four a D. Past D the ladder grows double letters: DD at five inches, DDD at six, then G, H, I, J and beyond. Some brands print the sixth step as F instead of DDD, which is the same cup with a different name.

The calculator measures from the band number, not the raw underbust. With an underbust of 31 and a band of 32, a 37 inch bust gives 37 minus 32, which is 5 inches and a DD cup. Measuring from the raw underbust instead would give 6 inches and a different letter whenever the band rounds, so use one method consistently.

Worked example: underbust 31 in, bust 37 in

Take the two numbers from the top of this page. Underbust 31 inches rounds to the nearest even value, and the tie between 30 and 32 goes up to 32. Bust 37 inches minus the 32 band leaves 5 inches, which is the DD step. The calculator therefore returns US 32DD as the center of the range.

The same body translates across systems like this. The UK keeps the 32 band and uses the same letter at this step. Europe labels the band in centimeters: 32 minus 4, times 2.5, gives a 70 band, and cups past D continue as single letters, so the cup reads E. France adds 15 to the European band number, giving 85E. Japan also uses centimeter bands in 5 cm steps and 2.5 cm cup steps, which lands on 70E here.

The sister sizes are 30DDD and 34D. If a 32DD band feels tight, 34D trades an inch of band for a smaller cup; if it gaps, 30DDD does the reverse. Every one of these labels can be found on a rack, and the wire that fits is the one that fits, whatever it says on the tag.

Sister sizes: the cup volume trick

A cup letter means nothing without its band. The cup of a 32DD holds roughly the same volume as the cup of a 30DDD and a 34D, because the letter scales with the band: going down two inches of band adds one cup letter to keep the same pocket of fabric. This chain of equivalents is why fitters tell you to try the sister sizes when the band is wrong.

Sister sizing also explains a common confusion: a woman wearing 36C and her friend wearing 32DD may be wearing cups of the same volume, with bands built for different ribcages. It is why a cup letter quoted without its band is close to meaningless in conversation.

The equivalent chain stops being exact at the extremes, where wire geometry starts to matter more than volume, but within the common bands it holds well enough to shop with.

Converting US to UK, EU, French and Japanese sizes

Ordering from a European or Japanese brand means converting twice: the band number and the cup letter each move on their own ladder. The band conversion between US even inches and European 5 cm bands follows one line: European band equals the US band minus 4, times 2.5. So US 32 becomes 70, US 34 becomes 75, US 30 becomes 65.

The UK ladder is the one that catches American shoppers. A US DDD is a UK E, and a US G is a UK F, because British brands keep single letters where Americans go double. Buy a UK 32DDD and you have bought a full cup larger than the calculator's 32DD.

Charts from individual brands deviate from this table by a letter at the edges of the range, and Japanese cups run in 2.5 cm steps rather than inches, so treat the conversion as the right neighborhood and confirm against the brand's own chart before paying for shipping.

What the standard and the studies say

There is a real standard behind European sizing. EN 13402-3, the European size designation standard, defines the cup as the difference between bust girth and underbust girth measured in centimeters, and the 2004 edition published a letter table that runs AA from 10 to 12 cm, A from 12 to 14, B from 14 to 16, C from 16 to 18 and D from 18 to 20. The 2017 revision loosened the fixed intervals so brands can keep their traditional size runs. Notice the mismatch with American practice: the centimeter ladder steps about 2 cm per letter while the US ladder steps a full inch, so letters do not map one to one at the extremes.

The fitting studies put numbers on how far practice sits from any system. The 2008 Chiropractic & Osteopathy study found 80 percent of its 30 participants incorrectly fitted, 70 percent in cups too small. The 2013 postmenopausal study measured 93 percent incorrectly sized. Both samples are small, so do not quote the percentages as population rates, but the direction is consistent across the literature: remembered sizes drift, and measurement resets them.

No American national standard fixes the US inch ladder, which is one reason double letters multiply and brands diverge. The honest summary is that the tape and the formulas get you to the correct shelf, and only trying the neighboring sizes settles it.

Fit checks that beat the number

The number is a starting point; these checks finish the fitting. Band: level all the way around, snug enough that two fingers slide under but the band does not lift when you raise your arms. Cups: no breast tissue spilling over the top or out the sides, and no wrinkling at the tip. Center gore: the strip between the cups should lie flat against the sternum. Straps: carrying weight only, not doing the band's job; if digging straps are the complaint, the band is usually the part that is too big.

When a size almost works, move within the sister chain rather than abandoning the letter system. A cup that wrinkles with a band that rides up is one size too big in both places, which is the single most common misfit pattern in the too-small-cup, too-big-band direction the studies describe.

Finally, expect variation between brands and even between styles from one brand, the same way shoe lasts differ. Buy one, wash it, wear it a day, then commit to the color. All calculations here run in your browser, and nothing you type is uploaded or stored. The sources behind this page are the 2008 bra fit study on PubMed, the 2013 postmenopausal fitting study, and a preview of the EN 13402-3 standard.